Assemblying structures in outer space has a number of associated problems. For instance, when joining structural truss members, the joint should allow insertion of a truss member into an existing precision structure. Distortions of more than a few thousandths of an inch are to be avoided.
The joint should also be sufficiently strong to minimize compliance due to compression of the joint over a large load range. The ability to operate at a variety of temperatures without losing the "preload" as a result of thermal expansion or contraction is also highly desirable. The "preload" is the compressive or tension force applied to a particular part or parts that must be overcome to move the part or to reduce the structural stability of the part. If the joint is unable to maintain its preload, the joint may fail.
Numerous attempts have been made to develop a simple, effective joint but many have had significant drawbacks. For instance, in one prior joint, the ends of the trusses have been provided with grooves for mating engagement with parts of a split lock ring. With the split lock ring placed in the grooves, a sleeve having an angled inner edge is screwed over the split lock ring. The angled edges cause the split lock ring to lock down tightly over the two interlocking ends of the trusses. An alternate design uses a lever and camming surfaces to tighten the joint. While some of these designs permit at least limited rotational alignment, this coupling method is very complicated. In addition, the separate parts such as the split lock ring typically are not attached to either of the trusses prior to assembly and therefore may easily be misplaced or lost. The joints also often have numerous angled exterior edges that may snag on equipment or space suits.
In another prior joint, the ends of the trusses have semi-circular grooves and wedges that form interlocking surfaces and prevent longitudinal displacement of either truss. A spring-loaded pivoting latch inside the joint prevents radial displacement to hold the ends together. While the joints are self-contained, the truss ends also tend to be complex, have several edges for potential snagging, and generally can only be attached in one specific rotational alignment. As a result, the astronaut connecting the joint usually must properly align the truss prior to connecting the joint or the joint cannot be connected.
Yet another prior joint uses a bayonet mount similar to those used for camera lenses. The end of one truss has flanges that are placed over the end of the other truss. A chuck key is used to tighten the flanges against ribs on the exterior wall of the second truss. However, the chuck key and the exterior edges considerably increase the risk of snagging and insertion of the truss usually temporarily distorts the existing structure. Also, the final rotational alignment is restricted to one of the rotational positions afforded by the position of the flanges. While the joint could be designed having several alignment positions, that design would likely require an equal number of flanges and ribs around the truss ends which could complicate and weaken the joint construction.
Finally, a joint having a ball-and-socket construction much like those of an automobile trailer can provide rotational alignment. However, the joint typically has reduced coupling strength and is subject to increased wear because of movement of the ball in the socket due to rotational, compressive, and expansive forces. The joint may also distort the existing structure when inserted and there is significant potential for snagging.
To minimize crew member fatigue, the joint should be easy to assemble. A temporary snap-action attachment prior to final tightening is very desirable to allow the astronaut to attach the two truss members without concern for alignment and thereafter the astronaut may align the two halves of the joint and create a solid structural attachment of the two trusses without undue force. Unfortunately, prior joints have not met all these objectives.